Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/20—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
  • H01S5/00—Semiconductor lasers
  • H01S5/20—Structure or shape of the semiconductor body to guide the optical wave ; Confining structures perpendicular to the optical axis, e.g. index or gain guiding, stripe geometry, broad area lasers, gain tailoring, transverse or lateral reflectors, special cladding structures, MQW barrier reflection layers
  • H01S5/2004—Confining in the direction perpendicular to the layer structure

Definitions

• the present invention relates to a semiconductor laser device having a structure in which a first conductivity type clad layer, an active layer, and a second conductivity type clad layer are sequentially stacked on a first conductivity type semiconductor substrate, Also, the present invention relates to a semiconductor laser device, a semiconductor laser module and an optical fiber amplifier which do not confine light having a wavelength different from that of emitted laser light.
• optical fiber communication information transmission on the Internet and the like has been performed by optical fiber communication.
• information is transmitted by transmitting an optical signal through an optical fiber.
• various devices have been devised to enable long-distance transmission of optical signals.However, it is impossible at present to control the optical intensity loss associated with long-distance transmission to zero, and a certain percentage of optical signals Decays. Therefore, an optical fiber amplifier for amplifying the attenuated optical signal is required.
• FIG. 12 (a) is a schematic diagram showing the structure of a backward pumping type optical fiber amplifier among the optical fiber amplifiers according to the conventional technology.
• the signal light emitted from the signal light source 101 is transmitted through the optical fiber, and is incident on the amplification optical fiber 104.
• the excitation light emitted from the excitation light source 102 is transmitted through the excitation light transmission optical fiber 105, passes through the coupler 103, and is incident on the amplification optical fiber 104.
• EDFA Erbium Doped Fiber Amplifier
• amplification light Erbium ions are added to the buffer 104, and the excitation light is amplified by:
• the signal light By being incident on 04, erbium ions are excited to a high energy state. Then, when the signal light enters the amplified optical fiber 104 in the excited state, Light having the same wavelength and the same phase as the signal light is stimulatedly emitted. Therefore, the signal light is amplified in intensity as compared with before being incident on the amplification optical fiber 104, and transmitted through the optical fiber as amplified signal light.
• the conventional optical fiber amplifier has a problem that certain noise is generated in the signal light due to the presence of the pump light source 102.
• the power blur 103 optically couples the optical fiber for transmitting the signal light and the pumping light transmission optical fiber 105. Therefore, as shown in Fig. 12 (b), not only the pump light is transmitted to the optical fiber, but also a part of the signal light indicated by the dotted arrow in Fig. 12 (b) is changed by the coupler 103. The light is branched and transmitted through the excitation light transmission optical fiber 105, and is incident on the excitation light source 102.
• the semiconductor laser device forming the excitation light source 102 has a resonator structure for performing laser oscillation. Specifically, in the semiconductor laser device constituting the excitation light source 102, a high reflection film is disposed on an end surface facing the end surface on the laser light emission side. For this reason, a part of the signal light that has been branched by the power blur 103 and incident on the semiconductor laser device is reflected by the highly reflective film, exits from the pump light source, and is transmitted through the pump light transmission optical fiber 105 to transmit power. The optical signal is again multiplexed with the signal light amplified by the bra 103.
• the resonator length of the pump light transmission optical fiber 105 and the pump light source 102 is provided.
• the signal light output from the power blur 103 contains noise consisting of a part of the signal light having a predetermined phase difference from the amplified signal light. Due to the presence of such noise, a signal reading error occurs on the signal light receiving side, and it becomes difficult to transmit information accurately.
• An object of the present invention is to provide a semiconductor laser device, a semiconductor laser module, and an optical fiber amplifier that suppress generation of noise caused by signal light that is once branched to a light source side. Disclosure of the invention
• the semiconductor laser device includes a first conductivity type semiconductor substrate, a first conductivity type cladding layer, an active layer, and a second conductivity type cladding layer sequentially laminated on the first conductivity type semiconductor substrate, and an emission side end face.
• a semiconductor laser device that emits a laser beam of a first wavelength from a side end surface, wherein the first-conductivity-type clad layer and the second-conductivity-type clad layer have a refractive index at a first wavelength, Having a value lower than the effective refractive index with respect to the wavelength of the second conductivity type, and for the light of the second wavelength that enters from the outside through the exit end face, the first conductivity type cladding layer or the second conductivity type
• the refractive index of at least one of the cladding layers is equal to or higher than the effective refractive index for the second wavelength.
• the refractive index of the first conductive type cladding layer and the second conductive type cladding layer is smaller than the effective refractive index for the first wavelength that is the emission wavelength.
• Light having a wavelength of 1 can be confined between the first conductivity type cladding layer and the second conductivity type cladding layer.
• the refractive index of at least one of the first conductivity type cladding layer and the second conductivity type cladding layer for the second wavelength is the effective refractive index.
• the semiconductor laser device according to the next invention is the semiconductor laser device according to the above invention, wherein the first conductivity type cladding layer and the second conductivity type cladding layer have a high impurity density, , Having a refractive index equal to or higher than the effective refractive index.
• the bending of the cladding layer having a high impurity density with respect to the second wavelength is performed.
• the refractive index is higher than the effective refractive index.
• the cladding layer can be made to have a higher refractive index than the effective refractive index by forming a cladding layer by thickly laminating a high-refractive-index material.
• the semiconductor laser device according to the next invention is the semiconductor laser device according to the above invention, wherein the first conductivity type cladding layer has a refractive index equal to or higher than the effective refractive index with respect to the second wavelength. This is a special feature.
• the refractive index for the second wavelength of the cladding layer of the first conductivity type is increased.
• the refractive index of the first conductivity type cladding layer is made higher than the effective refractive index, and the thickness of the active layer and the second conductivity type cladding layer is increased. If the structure is not increased, it is possible to provide a semiconductor laser device which does not adversely affect the conduction of heat generated from the active layer.
• the semiconductor laser device is the semiconductor laser device according to the above invention, wherein a first conductivity type cladding layer, an active layer, and a second conductivity type cladding layer are sequentially laminated on the first conductivity type semiconductor substrate, A semiconductor laser device having an emission side end surface and emitting a laser beam of a first wavelength from the emission side end surface, wherein a first wavelength of the first conductivity type clad layer and a second wavelength of the second conductivity type clad layer are provided.
• the refractive index of at least one of the first conductivity type cladding layer and the second conductivity type cladding layer is 1.03 times or less.
• the effective refractive index has a value equal to or larger than the refractive index of at least one of the cladding layers, if the effective refractive index is 1.03 times or less, the trapping of light incident from the outside is prevented. can do.
• the semiconductor laser device according to the next invention is the semiconductor laser device according to the above invention,
• the difference between the length and the second wavelength is not less than 200 nm.
• the difference between the first wavelength and the second wavelength is set to 20 O nm or more, light having the first wavelength is easily confined and light having the second wavelength is leaked. be able to.
• the semiconductor laser device according to the next invention is the semiconductor laser device according to the above invention, wherein the first wavelength is not less than 9500 nm and not more than 1100 nm, and the second wavelength is not more than 1100 nm.
• the feature is that it is not less than 500 nm and not more than 160 nm.
• the first wavelength is set to 950 nm or more and 1100 nm or less
• the second wavelength is set to 1500 nm or more and 1600 nm or less.
• this light is used as signal light and the light of the first wavelength is used as excitation light
• a semiconductor laser device can be provided that can leak inside even if signal light enters from the outside.
• a semiconductor laser device is characterized in that, in the above invention, the first wavelength is 980 nm, and the second wavelength is 550 nm. According to the present invention, since the first wavelength is 980 nm and the second wavelength is 155 O nm, a semiconductor laser device applicable to an excitation light source in optical communication is provided. be able to.
• the semiconductor laser device according to the next invention is the semiconductor laser device according to the above invention, wherein the first conductivity type waveguide layer disposed between the first conductivity type cladding layer and the active layer; A waveguide layer of the second conductivity type disposed between the cladding layer and the second conductivity type. .
• the waveguide layer of the first conductivity type and the waveguide layer of the second conductivity type are provided, the emitted laser light is confined in the active layer and the waveguide layer. Further, it is possible to provide a semiconductor laser device having a wider waveguide region as compared with the case where only the active layer is propagated, and having a durability against optical damage due to a reduction in optical output density.
• the semiconductor laser device is characterized in that, in the above invention, the active layer has a quantum well layer.
• the active layer since the active layer includes the quantum well layer, carriers can be efficiently confined in the quantum well layer by the quantum confinement effect, and the light emission efficiency can be improved.
• a semiconductor laser device is characterized in that, in the above invention, the active layer includes a plurality of quantum well layers and a barrier layer disposed between the plurality of quantum well layers.
• the carrier can be confined by the quantum confinement effect in the plurality of quantum well layers, and the temperature characteristics can be improved.
• the semiconductor laser device according to the next invention is the semiconductor laser device according to the above invention, wherein the second conductivity type cladding layer has a width in a direction perpendicular to a laser light emitting direction, the first conductivity type substrate. It is characterized by being narrower than the width.
• the width of the cladding layer of the second conductivity type is narrow, the density of the injected current can be improved, and a semiconductor laser device having high luminous efficiency can be provided.
• the semiconductor laser device according to the next invention is the semiconductor laser device according to the above invention, wherein a first conductivity type carrier block layer disposed between the first conductivity type waveguide layer and the active layer; A second conductive type carrier block layer disposed between the second conductive type waveguide layer and the second conductive type waveguide layer.
• the carrier block layer of the first conductivity type and the carrier blocking layer of the second conductivity type are provided, so that the injection carrier is confined in the vicinity of the active layer to suppress the carrier overflow and to reduce the carrier overflow.
• the thickness of the wave layer can be increased. This makes it possible to increase the output while maintaining the temperature characteristics.
• a semiconductor laser module according to the next invention includes the above-described semiconductor laser device, an optical fiber that guides laser light emitted from the semiconductor laser device to the outside, and optically couples the semiconductor laser device and the optical fiber. It is characterized by having an optical coupling lens system. According to the present invention, since the light having the second wavelength transmitted through the optical fiber is leaked inside the semiconductor laser device, the semiconductor laser module does not emit the light having the second wavelength again. Can be provided.
• the semiconductor laser module according to the next invention is the semiconductor laser module according to the above invention, further comprising: a photodetector that measures an optical output of the semiconductor laser device; a temperature control module that controls a temperature of the semiconductor laser device; and an isolator. It is characterized by having. According to the present invention, since the photodetector is provided, the intensity of the emitted laser light can be stabilized, and since the isolator is provided, the laser light emitted from the semiconductor laser device is again emitted. The return to the inside of the semiconductor laser device can be prevented.
• An optical fiber amplifier comprises: an excitation light source using the semiconductor laser device or the semiconductor laser module described above; an optical fiber transmitting signal light having the second wavelength; and an optical fiber connected to the optical fiber.
• the signal light even if a part of the signal light having the second wavelength enters the excitation light source via the power plastic, the signal light leaks inside the semiconductor laser device or the semiconductor laser module provided in the excitation light source. However, since it does not return to the optical fiber again, it is possible to suppress the generation of a noise component having the same wavelength as the signal light.
• An optical fiber amplifier according to the next invention is characterized in that, in the above invention, the amplification optical fiber is doped with erbium.
• FIG. 1 is a front view showing the structure of the semiconductor laser device according to the first embodiment.
• FIG. 2 is a cross-sectional view taken along line AA of the semiconductor laser device shown in FIG. Is
• FIG. 4 is a graph showing a refractive index distribution and an effective refractive index of the semiconductor laser device according to the first embodiment with respect to light having a wavelength of 150 nm, and FIG.
• FIG. 5B is a schematic diagram illustrating a mode of light leakage when light having a wavelength of 1550 nm is incident on the semiconductor laser device according to the first embodiment from the outside.
• FIG. 6 is a graph showing a refractive index distribution, a light intensity distribution, and an effective refractive index of the semiconductor laser device according to the first embodiment with respect to light having a wavelength of 980 nm.
• FIG. 6 shows a DCH laser of a comparative example.
• FIG. 7 is a graph showing a refractive index distribution, a light intensity distribution, and an effective refractive index for light having a wavelength of 980 nm in FIG. 7, and FIG. 7 has a wavelength of 550 nm in a DCH laser of a comparative example.
• FIG. 6 is a graph showing a refractive index distribution, a light intensity distribution, and an effective refractive index of the semiconductor laser device according to the first embodiment with respect to light having a wavelength of 980 nm.
• FIG. 8 is a graph showing a refractive index distribution, a light intensity distribution, and an effective refractive index with respect to light.
• FIG. 8 is a front view showing the structure of the semiconductor laser device according to the second embodiment.
• FIG. 8 is a cross-sectional view taken along the line BB of FIG. 8, and
• FIG. 11 is a side sectional view showing the structure of a semiconductor laser module according to Embodiment 3
• FIG. 11 is a schematic diagram showing the structure of an optical fiber amplifier according to Embodiment 4
• FIG. 1B and 1B are schematic diagrams showing a structure of an optical fiber amplifier according to a conventional technique and an aspect of light transmission.
• FIG. 1 is a front view showing the structure of the semiconductor laser device according to the first embodiment
• FIG. 2 is a side sectional view showing the structure of the semiconductor laser device according to FIG.
• the semiconductor laser device is a so-called Decoupled Confinement Heterostructure (hereinafter, referred to as “DCH”), which is a J-laser.
• DCH Decoupled Confinement Heterostructure
• it has a structure that includes a carrier layer that prevents carrier overflow in addition to a cladding layer that confines light.
• the semiconductor laser device according to the first embodiment includes an n-type substrate 1, an n-type cladding layer 2, an n-type waveguide layer 3, an n-type carrier block layer 4, an active layer 5, a p-type carrier block layer 6,
• the p-type waveguide layers 7 are sequentially stacked.
• An n-type current blocking layer 8 is disposed inside the p-type waveguide layer 7 except for a strip-shaped partial region whose longitudinal direction is parallel to the light emission direction. .
• a p-type cladding layer 9 On the p-type waveguide layer 7, a p-type cladding layer 9, a p-type contact layer 10, and a p-side electrode 11 are sequentially laminated.
• an n-side electrode 12 is arranged on the lower surface of the n-type substrate 1. Further, as shown in FIG. 2, a low-reflection film 19 is disposed on the light-emitting-side end surface (the right-side end surface in FIG. 2). Has a high reflection film 20 disposed thereon.
• the n-type cladding layer 2 and the p-type cladding layer 9 are for confining laser light generated by carrier recombination in the active layer 5. Further, the first embodiment has a function of not confining light having a predetermined wavelength other than the emitted laser light. This will be described in detail later.
• the n-type cladding layer 2 and the p-type cladding layer 9 are made of a material having a lower refractive index than the active layer 5, the n-type waveguide layer 3, and the P-type waveguide layer 7.
• the 11-type cladding layer 2 is 1. .. 45 0 &. 953
• the film thickness is 5.820 ⁇ 111
• the p-type cladding layer 9 is Al.
• the n-type waveguide layer 3 and the p-type waveguide layer 7 are for guiding laser light. As described above, in the DCH structure, it is not necessary to suppress carrier overflow by the cladding layer, so that the thickness of the n-type waveguide layer 3 and the p-type waveguide layer 7 can be increased. As a result, COD (Catastrophic Optical Damage) is suppressed and high light output is realized.
• the n-type waveguide layer 3 and the P-type waveguide layer 7 are composed of GaAs, and have a thickness of 0.470 ⁇ .
• the ⁇ -type carrier layer 4 and the ⁇ -type carrier layer 6 confine the injected carrier in the active layer 5 to suppress overflow of the carrier.
• the ⁇ -type carrier block layer 4 and the ⁇ -type carrier block layer 6 are made of a material having a bandgap larger than that of the active layer 5 and have a sufficiently small optical thickness.
• the ⁇ -type carrier block layer 4 prevents holes (holes), which are minority carriers in the ⁇ -type carrier block layer 4, from flowing out to the lower layer, and the ⁇ -type carrier block layer 6 prevents electrons from flowing out to the upper layer. Has been prevented.
• the ⁇ -type carrier block layer 4 and the ⁇ -type carrier block layer 6 are 1. . Consists 4 0 & 0. 6 3, the film thickness is respectively 0. Ru fc at 0 3 5 mu m.
• the active layer 5 is for generating a laser beam by recombination of the injected carriers. More specifically, as shown in the enlarged view of FIG. 1, the active layer 5 has two quantum well layers 14 and 16 formed by side barrier layers 13 and 17 and a barrier layer 15. It consists of a sandwiched structure. By having the quantum well layers 14 and 16, a quantum confinement effect is generated for the carriers, and the carriers can be confined with high efficiency.
• the quantum well layers 14 and 16 need to have an extremely small thickness in order to exhibit the quantum confinement effect, and the respective thicknesses in the first embodiment are 0.01 m.
• the composition of the quantum well layer 1 4, 1 6 consists of I n 0. 14 G a 0 . 86 A s.
• the side barrier layers 13 and 17 and the barrier layer 15 are composed of GaAs, respectively.
• the thickness of the side barrier layers 13 and 17 is 0.055 ⁇ m, and the thickness of the barrier layer 15 is 15 ⁇ m.
• the thickness is 0.006 ⁇ m.
• the active layer 5 is configured in such a manner, and the present embodiment
• the semiconductor laser device according to 1 emits a laser beam having a wavelength of 980 nm. Since the laser light is mainly guided through the n-type waveguide layer 3, the p-type waveguide layer 7, the n-type carrier block layer 4, the p-type carrier block layer 6, and the active layer 5, these are collectively referred to below. Then, it is called the waveguide region 18.
• the p-type contact layer 10 is for facilitating ohmic contact with the p-side electrode 11.
• the p-type contact layer 10 is formed by doping GaAs with little oxidation deterioration with a high concentration of a p-type impurity such as zinc (Zn).
• the thickness of the contact layer is 1.75 ⁇ m.
• the ⁇ -type current blocking layer 8 is for allowing the injected current to flow only to a desired region of the active layer 5. Further, by forming the ⁇ -type current blocking layer 8 from a material having a lower refractive index than the waveguide layer, an effective refractive index type waveguide can be formed. Thereby, an efficient single-mode semiconductor laser can be configured.
• the low reflection film 19 and the high reflection film 20 shown in FIG. 2 constitute a resonator.
• the highly reflective film 20 has a reflectance of 80% or more, more preferably 98% or more.
• the low-reflection film 19 has a film structure with a light reflectivity of 5% or less, preferably about 1%, in order to suppress instantaneous optical damage on the emission side end face.
• the light reflectance of the low reflection film 19 is optimized according to the cavity length.
• a fiber grating provided in an optical fiber that optically couples with the semiconductor laser device is included in the resonator together with the high reflection film 20.
• a distributed feedback laser structure in which a periodic groove structure is provided in a part of the ⁇ -type waveguide layer 3 or the ⁇ -type waveguide layer 7 of the semiconductor laser device, or a distributed Bragg reflection laser structure may be used.
• the semiconductor laser device according to the first embodiment has a function of having a wavelength different from the emission wavelength and leaking light incident from the outside inside the semiconductor laser device. This function is described below.
• light incident from the outside a case where light having a wavelength of 1550 nm is incident on the semiconductor laser device according to the first embodiment from the outside will be considered.
• FIG. 3 is a graph showing a refractive index distribution inside the semiconductor laser device according to the first embodiment and an effective refractive index of the semiconductor laser device.
• the solid line indicates the refractive index of each layer.
• the dotted line indicates the effective refractive index.
• a sectional view of the semiconductor laser device according to the first embodiment is attached below the graph so that the relationship between each layer and the refractive index distribution and the like becomes clear.
• the horizontal axis represents the lamination direction of each layer constituting the semiconductor laser device according to the first embodiment, and the origin of the horizontal axis is set at the boundary between the n-type cladding layer 2 and the n-type waveguide layer 3. are doing.
• the refractive index of each layer shown in FIG. 3 indicates the refractive index for light having a wavelength of 1550 nm.
• the refractive index of the n-type cladding layer 2 is 3.413 for light having a wavelength of 1550 nm
• the refractive indexes of the n-type waveguide layer 3 and the p-type waveguide layer 7 are 3. 436.
• the refractive index of the p-type cladding layer 9 is 3.274.
• the refractive index of the n-type carrier block layer 4 and the p-type carrier block layer 6 is 3.233, the side barrier layers 13 and 17 are 3.436, and the quantum well layers 14 and 16 are 3 It is 481.
• the refractive index distribution in FIG. 3 is shown based on these values.
• the effective refractive index shown in FIG. 3 can be derived from a specific structure such as the thickness and the refractive index of each layer constituting the semiconductor laser device. The derivation is described below.
• the light intensity distribution and the effective refractive index are obtained from the solution and eigenvalue of a predetermined wave equation.
• the wave equation for deriving the light intensity distribution and the effective refractive index is
• V t 2 + ⁇ k 0 2 n 2 (x, y)- ⁇ 2 ⁇ ] E (x, y) 0 ⁇ (1)
• the laser beam emission direction is the z-axis
• the direction in which the layers are stacked ie, the direction of the horizontal axis in FIG. 3
• V t is (3Z3 X , 3 / d y) 0
• E (x, y) indicates the electric field vector at coordinates (x, y).
• n (X, y) indicates the refractive index at coordinates (X, y), and k. Is the wave number Is shown.
• ⁇ is the eigenvalue of the electric field vector E (x, y) in Eq. (1). Since the structure of the semiconductor laser device is uniform in the z direction, the electric field vector E (x, y) and the refractive index n (x, y) do not depend on the z coordinate.
• the electric field vector E (X, y) is obtained, and the electric field vector E (X, y) is obtained.
• the light intensity distribution is obtained from the intensity distribution of E (X, y).
• FIG. 3 does not show the light intensity distribution.
• the light with a wavelength of 1550 nm is obtained.
• the effective refractive index is 3.413.
• the semiconductor laser device according to the first embodiment has a structure in which the refractive index of the n-type cladding layer 2 has the same value as the effective refractive index for light having a wavelength of 1550 nm.
• the refractive index of the cladding layer is actually When the semiconductor laser device has a value equal to or higher than the effective refractive index, the semiconductor laser device does not confine light having a predetermined wavelength in the waveguide region 18.
• FIGS. 4 (a) and 4 (b) show that the semiconductor laser device according to the first embodiment has a wavelength of 150 nm externally transmitted through the low-reflection film 19 with respect to the semiconductor laser device according to the first embodiment.
• FIG. 3 is a schematic diagram showing a state of light transmission inside a semiconductor laser device when light is incident.
• the semiconductor laser device according to the first embodiment has a structure in which the light of 550 ⁇ m is not confined inside the waveguide region 18. Having. Therefore, the light incident from the outside leaks toward the n-type cladding layer 2 as it travels.
• the light emitted from the low-reflection film 19 has little or no intensity compared with the incident light.
• the semiconductor laser device when light having a wavelength of 1550 nm, which is a wavelength different from the emission laser wavelength, is incident, the light is reflected by the high-reflection film 20, and the light is reduced. There is an advantage that it is possible to suppress emission from the reflection film 19.
• the semiconductor laser device according to the first embodiment cannot function as a laser light emission light source. That is, it is the wavelength of the laser light emitted from the semiconductor laser device. It is necessary that the light of the wavelength of 980 nm be confined inside the waveguide region 18.
• the semiconductor laser device according to the first embodiment performs light confinement inside the waveguide region 18 with respect to light of 980 nm.
• FIG. 5 is a graph showing a light intensity distribution, a refractive index distribution, and an effective refractive index of the semiconductor laser device according to the first embodiment with respect to the light of 980 nm, which is the wavelength of the emitted laser light.
• the refractive index changes depending on the wavelength.
• the refractive index of each layer for light having a wavelength of 980 nm is 3.511 for the n-type cladding layer 2 and 3 for the n-type waveguide layer 3 and the p-type waveguide layer 7, respectively.
• the n-type carrier block layer 4 and the p-type carrier block layer 6 are 3.306.
• the refractive index of the p-type cladding layer 9 is 3.352.
• the refractive index of the side barrier layers 13 and 17 and the barrier layer 15 constituting the active layer 5 is 3.537, and the quantum well layers 14 and 16 are 3.591. .
• the light intensity distribution can be obtained by solving the wave equation (1) for the electric field vector E (X, y).
• the effective refractive index can also be obtained by solving equation (1) and equation (2), which shows that the effective refractive index for light at a wavelength of 980 nm is 3.515. .
• the semiconductor laser device in order for the emitted laser light to propagate only in the waveguide region 18, the semiconductor laser device according to the first embodiment requires the refractive indexes of the n-type cladding layer 2 and the p-type cladding layer 9. Has a low value. However, light confinement does not occur at any value as long as the value is lower than the refractive index of the active layer 5, the n-type waveguide layer 3, and the P-type waveguide layer 7. In general, when the refractive indices of the n-type cladding layer 2 and the p-type cladding layer 9 are lower than the effective refractive index of the semiconductor laser device, the semiconductor laser device can perform optical confinement. is there.
• FIGS. 6 and 7 show the light intensity distribution, the refractive index distribution, and the effective refractive index of the conventional DCH laser.
• FIG. 6 is a graph showing a refractive index distribution and a light intensity distribution for light of 980 nm, which is the emission wavelength of a conventional DCH laser
• FIG. 7 is a graph showing a refractive index distribution and light intensity for 1550 nm light. It is a graph which shows a light intensity distribution.
• the light intensity distribution and the effective refractive index are obtained from Equations (1) and (2), as in the case of FIG.
• the conventional DCH laser has basically the same structure as the semiconductor laser device according to the first embodiment, but has an 11-type cladding layer. .. 9 0 &. . 91 consists of three, and that it is a film thickness of 2. 5 m, the film thickness of the p-type cladding layer is different in a 0. 845 ⁇ .
• the optical characteristics are such that the refractive index and the effective refractive index of the ⁇ -type cladding layer for each wavelength are different from those of the semiconductor laser device according to the first embodiment.
• the refractive index of the n-type cladding layer is 3.484, and the effective refractive index is 3.509 from equations (1) and (2).
• the refractive index of the n-type cladding layer is 3.391, and the effective refractive index is 3.401.
• the refractive index of both the n-type cladding layer and the p-type cladding layer has a value smaller than the effective refractive index for the emission wavelength of 980 nm. Then, even for a wavelength of 1550 nm, the refractive indexes of both the n-type cladding layer and the p-type cladding layer are smaller than the effective refractive index. Since the refractive index of the n-type cladding layer is smaller than the effective refractive index at both wavelengths, the conventional DCH laser has an optical confinement effect not only at 980 nm but also at 1550 nm. Is considered to occur.
• a conventional DCH laser for example, as an excitation light source for an optical fiber amplifier
• part of the signal light having a wavelength of 1550 nm is incident on the conventional DCH laser, and the DCH laser Then, the light is reflected by the reflection-side end face and is emitted again.
• the semiconductor laser device according to the first embodiment does not have such a problem because light having a wavelength of 155 O nm leaks as described above.
• the semiconductor laser device can confine the light of 980 nm, which is the emission wavelength, in the waveguide region to such an extent that there is no practical problem compared with the conventional DCH laser.
• Laser oscillation can be performed with the same efficiency as a conventional DCH laser.
• the semiconductor laser device can effectively leak light incident from the outside, and Either 50 nm light is not emitted from the emission end face, or even very weak light is emitted.
• the semiconductor laser device according to the first embodiment when used as an excitation light source for an optical fiber amplifier such as an EDFA, it is possible to suppress generation of a noise component having the same wavelength as the signal light. it can. Further, regardless of the presence or absence of light incident from the outside, laser light of a predetermined wavelength can be emitted stably. Further, in the semiconductor laser device according to the first embodiment, as described above, the film thickness of the n-type cladding layer 2 is increased as compared with the conventional DCH laser. In general, it is known that the electrical resistance and the thermal resistance inside the semiconductor laser device increase as the film thickness increases, but this does not cause any particular problem in the first embodiment.
• the semiconductor laser device according to the first embodiment is not particularly inferior in terms of electric resistance and thermal resistance as compared with the conventional DCH laser. Further, in the semiconductor laser device according to the first embodiment, since the aluminum composition of the n-type cladding layer 2 is reduced as compared with the conventional DCH laser, electric resistance and thermal resistance can be suppressed.
• the p-type cladding layer 9 is formed of the same semiconductor layer as that of the conventional DCH laser, and has a small thickness. With such a structure, the effective refractive index for light having a wavelength of 1550 nm is reduced, but it also has a secondary effect.
• the semiconductor laser device is usually fixed on a mount having good thermal conductivity, and suppresses a rise in temperature of the active layer 5 by releasing generated heat to the mount.
• the semiconductor laser device is fixed on the mount by a so-called junction-down configuration, and the p-side electrode 11 is in contact with the mount.
• the semiconductor laser device according to the first embodiment since the thickness of the p-type cladding layer 9 is small, the heat generated in the active layer 5 can be efficiently discharged.
• the wavelength of light incident from the outside is 1550 nm.
• the wavelength of light is not limited to 1550 nm. It is not necessary to interpret the emission wavelength of the semiconductor laser device to be limited to 980 nm. Whether or not light incident from the outside leaks inside the semiconductor laser device is determined only by the correlation between the refractive index of the cladding layer and the effective refractive index, and has no relation to the emission wavelength of the semiconductor laser device. It is. That is, the structure of the semiconductor laser device may be designed such that the effective refractive index of the semiconductor laser device is equal to or lower than the refractive index of the cladding layer for incident light having a predetermined wavelength. .
• the wavelength of the light incident from the outside and the emission wavelength of the semiconductor laser device are close to each other, it is not easy to leak the light incident from the outside, so the difference between the wavelength of the incident light and the emission wavelength is difficult.
• the value is preferably at least 200 nm.
• the emission wavelength of the semiconductor laser device is set to 950 nm to 110 nm and the wavelength of light incident from the outside is set to 150 nm to 160 nm, the output laser light Thus, it is possible to easily leak light incident from the outside while ensuring sufficient light confinement. ''
• the structure of the semiconductor laser device is determined so that the refractive index of the n-type cladding layer 2 becomes the same value as the effective refractive index with respect to light incident from the outside.
• the structure is not limited to a structure in which the refractive index of the cladding layer 2 and the effective refractive index have the same value. Even when the refractive index of the n-type cladding layer 2 with respect to the wavelength of the incident light is higher than the effective refractive index, the incident light can be leaked. Further, the composition and thickness of each layer are not limited to the above values.
• the structure of the semiconductor laser device can be changed according to the use of the semiconductor laser device, the manufacturing cost, and the like.
• the refractive index of the p-type cladding layer 9 for incident light is equal to or smaller than the effective refractive index. Even if the semiconductor laser device is designed to have a high value, it is good.
• the semiconductor laser device may be configured such that the refractive indexes of both the n-type cladding layer 2 and the p-type cladding layer 9 are higher than the effective refractive index. In this case, outside Since the light incident from the part leaks not only to the n-type cladding layer 2 but also to the ⁇ -type cladding layer 9, light incident from the outside can be attenuated more effectively.
• the effective refractive index with respect to light incident from the outside is reduced by changing the mixed crystal ratio and the film thickness of the semiconductor mixed crystal constituting the ⁇ -type cladding layer 2 and the ⁇ -type cladding layer 2 is formed.
• the present invention is not limited to this structure.
• the material of the ⁇ -type waveguide layer 3 or the ⁇ -type waveguide layer 7 may be changed to lower the effective refractive index. That is, the semiconductor laser device according to the first embodiment can be realized even if the refractive index of the cladding layer can be increased as compared with the conventional laser device, or even if the effective refractive index of the semiconductor laser device is suppressed. Can be.
• the composition and the thickness of the active layer 5 and the ⁇ -type waveguide layer 3 other than the cladding layer may be adjusted to reduce the effective refractive index.
• a structure may be adopted in which, when a plurality of lights of different wavelengths or light of a certain wavelength band enters from the outside, the light leaks inside the semiconductor laser device.
• Such a semiconductor laser device can be realized by a configuration in which the refractive index of the cladding layer at each wavelength is lower than the effective refractive index.
• the refractive index of the ⁇ -type cladding layer 2 is lower than the effective refractive index with respect to light incident from the outside, the difference is small, and the ⁇ -type cladding layer 2 does not affect the emission wavelength. If the refractive index of the ⁇ -type cladding layer is lower than the effective refractive index, light entering from the outside is hardly confined in the waveguide region and does not exit from the exit end face. Even when emitted, only very weak light is emitted. However, as can be seen from FIG. 7 showing the refractive index distribution and the like for the light having a wavelength of 150 nm in the DC laser of the comparative example, the effective refractive index is 1.00 of the refractive index of the cladding layer 2.
• the value of the effective refractive index with respect to light incident from the outside is within 1.03 times the refractive index of the cladding layer 2.
• the semiconductor material forming each layer is not limited to the above.
• the conductivity type of the substrate may be not only n-type but also p-type.
• a structure in which a mold clad layer, a p-type waveguide layer, and the like are sequentially stacked may be employed.
• FIG. 8 is a schematic sectional view showing the structure of the semiconductor laser device according to the second embodiment
• FIG. 9 is a sectional view taken along line BB of FIG.
• the semiconductor laser device according to the second embodiment has a so-called ridge structure, and the structure of the semiconductor laser device according to the second embodiment will be described below.
• semiconductor layers similar to those in the first embodiment have the same names, and have the same functions unless otherwise specified.
• the semiconductor laser device includes an n-type cladding layer 22, an n-type waveguide layer 23, an active layer 24, and a p-type waveguide
• the layer 25 and the p-type cladding layer 26 are laminated.
• the p-type cladding layer 26 is processed into a mesa stripe in the upper region, and the width of the upper region of the p-type cladding layer 26 in the direction perpendicular to the laser light emission direction is equal to that of the n-type substrate 21. It is narrower than the width.
• a p-type contact layer 27 is laminated, and most of the upper surface of the p-type cladding layer 26 and the p-type contact layer 27 are formed by the insulating layer 28. Covered.
• the portion of the upper surface of the p-type cladding layer 26 that is not covered with the insulating layer 28 is a stripe-shaped region having a longitudinal direction in the laser light emission direction.
• a p-side electrode 29 is disposed on the insulating layer 28 and the exposed p-type contact layer 27.
• an n-side electrode 30 is disposed on the lower surface of the n-type substrate 21.
• the active layer 24 has a so-called multiple quantum well structure, and includes a barrier layer 31a, a quantum well layer 32a, a barrier layer 31b, a quantum well layer 32b, and a barrier layer 31 in order from the bottom. c, the quantum well layer 32c, and the barrier layer 31d are laminated.
• the insulating layer 28 is for allowing a current injected from the p-side electrode 29 to flow only in a partial region inside the semiconductor laser device.
• the current injected from the p-side electrode 29 due to the presence of the insulating layer 28 causes the insulating layer 28 to exist on the upper surface of the p-type contact layer 27. It flows into the inside of the semiconductor laser device only from the not-partially striped region. By narrowing the region where the current flows, high-density current can be injected and the luminous efficiency is improved. Therefore, the insulating layer 28 has the function of a current blocking layer, and need not be formed of an insulating material as long as it functions as a current block layer. For example, even when the semiconductor device is formed of an 11-type semiconductor layer, the inflow of current can be prevented.
• the refractive index of at least one of the n-type cladding layer 22 and the p-type cladding layer 26 is equal to the effective refractive index with respect to the wavelength of light incident from the outside.
• the semiconductor laser device may be designed to be higher than the effective refractive index. Then, with respect to the light having the wavelength of the emitted laser light, the refractive index of both the n-type cladding layer 22 and the p-type cladding layer 26 is designed to be lower than the effective refractive index.
• the semiconductor laser device according to the second embodiment is configured such that light having an emission wavelength is confined in the waveguide region, and light incident from outside is confined to the outside of the waveguide region.
• a leaked semiconductor laser device can be realized.
• the structure is such that the refractive indexes of the n-type cladding layer 22 and the p-type cladding layer 26 are lower than the effective refractive index.
• the refractive index of the p-type cladding layer 26 In the case where a material having a high refractive index is used for the cladding layer and the refractive index of the cladding layer is made higher than the effective refractive index by increasing the film thickness, as in Embodiment 1, In this case, it is desirable to increase the refractive index of the p-type cladding layer 26.
• the thermal resistance and electric resistance of the clad layer increase by increasing the thickness of the clad layer having a high refractive index.
• the semiconductor laser device according to the second embodiment has a ridge structure and is fixed on a mount, the n-side electrode 30 is fixed so as to be in contact with the upper surface of the mount.
• the mount has a function as a heat sink that emits heat generated by the semiconductor laser device to the outside. Therefore, the mount is located between the active layer 24 and the n-side electrode 30. It is not advisable to increase the thermal resistance of the doped layer 22.
• the n-type cladding layer 22 can maintain the same thickness as the conventional one, and the heat radiation efficiency does not deteriorate. .
• the density of impurities doped into the p-type cladding layer 26 must be increased in order to suppress an increase in electric resistance. Desired ,.
• the present invention is not limited to these two structures, and the semiconductor laser device to which the present invention is applied can be applied to a separate confinement structure (SCH) laser, and can be applied to a semiconductor laser device having another structure. There may be. More specifically, the present invention can be applied to any semiconductor laser device having a structure in which an active layer for performing light emission recombination of a carrier is sandwiched between cladding layers having a lower refractive index than the active layer.
• SCH confinement structure
• a semiconductor laser module is configured using the semiconductor laser device according to the first or second embodiment.
• FIG. 10 is a side sectional view showing a configuration of a semiconductor laser module according to Embodiment 3 of the present invention.
• the semiconductor laser module according to the third embodiment includes a semiconductor laser device 41 corresponding to the semiconductor laser device described in the first embodiment.
• the semiconductor laser device 41 has a junction-down configuration in which the p-side electrode is joined to the laser mount 48.
• a temperature control module 50 as a temperature control device is arranged on an inner bottom surface of a package 51 formed of ceramic or the like.
• a base 47 is disposed on the temperature control module 50, and a laser mount 48 is disposed on the base 47. It is.
• the temperature control module 50 is supplied with a current (not shown), and performs cooling and heating depending on the polarity.
• the temperature control module 50 cools and controls the temperature to a lower temperature, and the laser light has a shorter wavelength than the desired wavelength. In some cases, it is heated to a higher temperature.
• This temperature control is specifically controlled based on a detection value of a thermistor 49 disposed on the laser mount 48 and in the vicinity of the semiconductor laser device 41.
• the temperature control module 50 is controlled so that the temperature of the laser mount 48 is kept constant.
• a control device (not shown) controls the temperature control module 50 so that the temperature of the laser mount 48 decreases as the drive current of the semiconductor laser device 41 increases. By performing such temperature control, the output stability of the semiconductor laser device 41 can be improved, which is also effective for improving the yield.
• the laser mount 48 is desirably formed of a material having high thermal conductivity, such as a diamond. This is because, when the laser mount 48 is formed of diamond, heat generation when a high current is applied is suppressed. '
• Laser light emitted from the semiconductor laser device 41 is guided to the optical fiber 45 via the first lens 42, the isolator 43, and the second lens 44.
• the second lens 44 is provided on the package 51 on the optical axis of the laser beam, and is optically coupled to an optical fiber 45 connected externally.
• the current monitor 46 monitors and detects light leaked from the high reflection film side of the semiconductor laser device 41.
• an isolator 43 is interposed between the semiconductor laser device 41 and the optical fiber 45 so that reflected return light from other optical components does not return into the resonator. ing.
• the semiconductor laser device 41 has the structure shown in FIGS. 1 and 2, a fiber grating is disposed inside the optical fiber 45, and the semiconductor laser device 4
• the structure is such that the resonator is formed with the reflection-side end face of No. 1.
• the isolators 43 need to be in-line instead of being arranged in the semiconductor laser module.
• the semiconductor laser device according to the first embodiment is used as the semiconductor laser device 41.
• the semiconductor laser device 41 When light having a wavelength different from the wavelength of the laser light emitted from the semiconductor laser device 41 is transmitted through the optical fiber 45, the light passes through the second lens 44 and the first lens 42 and passes through the semiconductor laser device. 4 It is incident on 1. Since the light incident on the semiconductor laser device 41 is diffused, it is suppressed that the light is reflected by the reflection-side end face of the semiconductor laser device 41 and is again emitted from the semiconductor laser device 41 to the optical fiber 45. can do. Further, as described in the first embodiment, light incident from the outside leaks from the waveguide region of the semiconductor laser device to the n-type cladding layer. Since the semiconductor laser device 41 is fixed on the laser mount 48 in a junction-down configuration, light incident from an external force leaks vertically upward.
• the light incident from the outside is emitted to the upper part of the semiconductor laser device 41. If the light is emitted to the lower part, the light may be reflected at the boundary with the laser mount 48 and return to the semiconductor laser device 41 again, but this is not the case by being emitted to the upper part. Further, since the light is emitted upward, the emitted light does not impede the current monitor 46 and does not hinder the wavelength monitoring.
• the semiconductor laser device according to the second embodiment may be used as the semiconductor laser device 41. Since the semiconductor laser device according to the second embodiment has a ridge structure, when the semiconductor laser device is fixed on the laser mount 48, the n-side electrode and the laser mount 48 contact each other instead of having a junction-down structure. It is preferable to adopt an embodiment in which
• FIG. 14 is a schematic diagram illustrating a structure of an optical fiber amplifier according to a fourth embodiment.
• the optical fiber amplifier according to the fourth embodiment includes a semiconductor laser module 55 functioning as an excitation light source, an amplification optical fiber 59 for amplifying the signal light 56, and an excitation light emitted from the semiconductor laser module 55. And a WDM force brass 58 for causing the light to enter the amplification optical fiber 59. Further, an isolator 57 is arranged before the signal light 56 enters the WDM force blur 58, and an isolator 60 is arranged after the amplification optical fiber 59.
• the signal light 56 is light emitted from the signal light source and transmitted through the optical fiber, and has a wavelength of 155 nm.
• the WDM coupler 58 outputs the pump light emitted from the semiconductor laser module 55 to the amplification optical fiber 59. Further, the isolator 57 blocks light reflected from the WDM force bra 58 and suppresses noise and the like. Further, the isolator 60 is for shielding the amplification optical fiber 59 from reflected light.
• an erbium-doped optical fiber is used for the optical fiber 59 for width.
• EDF is made by adding erbium ions (Er3 + ) to an optical fiber, and absorbs light with a wavelength of about 980 nm or about 148 nm to excite electrons in erbium ions. It has the property to be. These electrons amplify the signal light 56 having a wavelength of 150 nm.
• the semiconductor laser module 55 uses the semiconductor laser module according to the third embodiment. Accordingly, light having a wavelength of 150 nm, which is incident on the semiconductor laser module 55 from the outside, diffuses in the semiconductor laser module 55. Then, the light of 150 nm is not emitted again from the semiconductor laser module 55 or emitted with a small light intensity that does not affect the signal light.
• the laser light emitted from the semiconductor laser module 55 as an excitation light source passes through the WDM coupler 58 and is incident on the amplification optical fiber 59 from the front. Since the wavelength of the incident laser light is 980 nm, the laser light No. 59 is absorbed by the erbium ions doped in the semiconductor, and the electrons in the erbium ions are excited.
• the signal light 56 passes through the isolator 57 and enters the amplification optical fiber 59 from behind. As described above, the electrons of the Erbium ions doped in the amplification optical fiber 59 are excited, and the signal light 56 is amplified by the energy of the excited electrons.
• a part of the amplified signal light 56 is branched by the WDM coupler 58 and enters the semiconductor laser module 55.
• the semiconductor laser device mounted on the semiconductor laser module 55 leaks 150 nm light into the n-type cladding layer without confining it in the waveguide region.
• the structure has a structure to release the light to the substrate. Therefore, most of the light incident on the semiconductor laser module 55 does not reach the reflection-side end face, and most of the light that reaches the reflection-side end face and leaks to the n-type cladding layer, It will not be emitted again.
• the light having a wavelength of 980 nm, which is the emission wavelength is confined in the waveguide region, so that the function of the pump light as an output light source is not impaired.
• the optical fiber amplifier according to the fourth embodiment can effectively suppress noise components having the same wavelength as the signal light. Therefore, the original signal light and the noise component having a phase difference from the original signal light by passing through the semiconductor laser module 55 are not multiplexed, which may hinder information transmission. Absent.
• the fourth embodiment employs a so-called forward pumping method in which pumping light is pumped from the front of the amplification optical fiber 59
• the present invention is not limited to this method.
• the present invention can be applied to an optical fiber amplifier of a so-called backward pumping system in which signal light and amplification light before amplification are multiplexed in advance and then input to the amplification optical fiber 59. It is possible.
• the signal light is amplified using the EDFA.
• the present invention can be applied to an optical fiber amplifier using a pumping method other than the EDFA.
• the semiconductor laser device included in the pump light source has a structure in which signal light is diffused, so that noise components can be suppressed.
• the semiconductor laser module according to the third embodiment is used as an excitation light source, but can be applied to a signal light source.
• a semiconductor laser module incorporating a semiconductor laser device is used as a signal light source in the same manner as an excitation light source. For this reason, it is necessary to prevent the light incident from the outside from being reflected on the reflection-side end face of the semiconductor laser device and entering the optical fiber again.
• the refractive indices of the first conductivity type cladding layer and the second conductivity type cladding layer are larger than the effective refractive index. Since it is small, it has an effect that light of the first wavelength can be confined between the cladding layer of the first conductivity type and the cladding layer of the second conductivity type.
• the refractive index of at least one of the first conductivity type cladding layer and the second conductivity type cladding layer for the second wavelength is different. Since it has a value equal to or more than the effective refractive index, there is an effect that light entering from the outside is not confined and light can be leaked to the cladding layer having a high refractive index.
• the refractive index of the clad layer having a high impurity density for the second wavelength is higher than the effective refractive index.
• the cladding layer can be made to have a higher refractive index than the effective refractive index by forming a cladding layer by laminating a high-refractive-index material thickly.
• the refractive index of the first conductivity type cladding layer is made higher than the effective refractive index, and the thickness of the active layer and the second conductivity type cladding layer are increased. If the structure is not increased, it is possible to provide a semiconductor laser device which does not adversely affect the conduction of heat generated from the active layer.
• the difference between the first wavelength and the second wavelength is set to 200 nm or more, the light having the first wavelength is easily confined, and the light having the second wavelength is easily leaked. This has the effect of being able to leak.
• the first wavelength is not less than 950 nm and not more than OO nm
• the second wavelength is not less than 1500 nm and not more than 160 nm.
• the first wavelength is 980 nm and the second wavelength is 155 nm, so that the semiconductor laser can be applied to an excitation light source in optical communication. The effect that a device can be provided is produced.
• the first conductive type waveguide layer and the second conductive type waveguide layer are provided, so that the emitted laser light is confined in the active layer and the waveguide layer.
• the semiconductor laser device has a wider waveguide region than the case where only the active layer is propagated, and can reduce the optical output density, thereby providing a semiconductor laser device having durability against optical damage.
• the active layer includes the quantum well layer
• carriers can be efficiently confined in the quantum well layer due to the quantum confinement effect, and light emission efficiency can be improved. This has the effect.
• the carrier can be confined by the quantum confinement effect in the plurality of quantum well layers. There is an effect that the temperature characteristics can be improved.
• the width of the cladding layer of the second conductivity type is narrow, the density of the injected current can be improved, and a semiconductor laser device having high luminous efficiency can be provided. When it can be done, it has a ray effect.
• the injection carrier is confined near the active layer to suppress the carrier overflow.
• the semiconductor laser since the light having the second wavelength transmitted through the optical fiber is diffused inside the semiconductor laser device, the semiconductor laser which does not emit the light having the second wavelength again This has the effect of providing a module.
• the photodetector since the photodetector is provided, the intensity of the emitted laser light can be stabilized, and since the isolator is provided, the laser light emitted from the semiconductor laser device is again used as a semiconductor. This has the effect of preventing return to the inside of the laser device.
• the signal light is diffused inside the semiconductor laser device or the semiconductor laser module provided in the excitation light source.
• the effect of suppressing the generation of noise components having the same wavelength as the signal light can be achieved.
• erbium is added to the amplification optical fiber, so that an optical amplifier using EDFA can be provided.
• the semiconductor laser device, the semiconductor laser module, and the optical fiber amplifier using the semiconductor laser module according to the present invention realize stable and high gain amplification and can be used for an optical communication system.

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